1. Field of the Invention
The present invention relates to a radar device, and in particular, to an frequency modulated continuous-wave (FN-CW) radar device that detects a plurality of objects.
2. Description of Related Art
As a vehicle-mounted radar device that can measure a distance and relative velocity of an object, an FM-CW radar device is known (e.g., see Patent document 1.) FIG. 1 illustrates a configuration diagram of a conventional FM-CW radar device. Based on transmitting a signal from a voltage controlled oscillator (VCO) 4, an electric wave is transmitted from an antenna 1 via a transceiver 2 to an object (not shown) and then, a reflected wave from the object is received by antenna 1 so that a received signal is generated by transceiver 2. The transmitting signal and the received signal are mixed by a mixer 3 and passed through a low pass filter (LPF) 5 to generate a beat signal. The beat signal is converted to a digital signal by an A/D converter 6. After that, this digital signal is Fourier transformed by an up-sweep Fourier transformer 7 and a down-sweep Fourier transformer 8 and, then input to a signal processor 9 that calculates data values such as distance, velocity etc., of the object. Signal processor 9 comprises a central processing unit (CPU) 91 and a memory 92.
The FM-CW method will be briefly described. First, an up-sweep signal that is frequency-modulated so that its frequency increases linearly and a down-sweep signal that is frequency-modulated so that its frequency decreases linearly are alternately transmitted to an object. FIG. 2 is a waveform diagram illustrating variation with time of transmitting and receiving waves in a radar device. In this figure, the transmitting signal is indicated by solid lines and the received signal is indicated by dotted lines.
A part of the signal reflected from the object and captured by an antenna and the transmitting signal is mixed to obtain a beat signal. Frequencies of the beat signal during the up-sweep and down-sweep periods are given by equations (1) and (2), respectively:
                              f          up                =                                            -                                                2                  ⁢                  Δ                  ⁢                                                                          ⁢                  f                                                  cT                                      m                    ⁢                                                                                                                                      ⁢            R                    -                                    2              λ                        ⁢            v                                              (        1        )                                          f          down                =                                                            2                ⁢                Δ                ⁢                                                                  ⁢                f                                            cT                m                                      ⁢            R                    -                                    2              λ                        ⁢            v                                              (        2        )            where Δf is a modulation frequency width; Tm is a modulation repetition period, c is a speed of light, and λ is a wavelength. By solving the simultaneous equations (1) and (2), the distance R from the radar to the object and the relative velocity v of the object can be obtained.
When there are a plurality of objects and reflected waves from the plurality of objects received simultaneously, pairs of the simultaneously detected up and down frequencies have to be determined. Generally, a plurality of moving objects may be observed simultaneously by the radar. For example, in an urban area etc., a large number of reflected signals are received simultaneously. Peaks (frequency peaks) of the up and down frequencies are therefore detected at a plurality of frequency values. Further, in the signals resulting from frequency analysis, frequency peaks due to noise appear. By way of example, FIGS. 3A and 3B illustrate frequency dependence of Fast Fourier Transform (FFT) outputs of the up and down frequencies detected from the reflected waves from a plurality of objects, respectively. It can be seen that the up frequency (FIG. 3A) and the down frequency (FIG. 3B) have a plurality of peaks, respectively.
Thus, in the conventional signal processing method, it may be difficult to determine pairs (hereinafter referred to as “pairing”) of the up and down frequencies, while results in of paring between incorrect peaks (hereinafter referred to as “mispairing”) i.e., erroneous calculation results such as a distance from the radar to a moving object and a moving velocity of the object.
[Patent document 1] JP-A-2004-93242